A. Technical Field
The present invention relates to a production process for a hydrophilic polymer such as a water-soluble or water-swellable polymer, and, in more detail, to a process for producing a hydrophilic polymer having a low residual monomer content while retaining the properties of a hydrogel polymer resultant from polymerization.
B. Background Art
Hydrophilic polymers are represented by water-soluble polymers and water-swellable polymers. Examples of the water-soluble polymer include: sodium polyacrylate; partially hydrolyzed products of polyacrylamide; and polyvinyl alcohol. These water-soluble polymers are, for example, used for: flocculants for water treatment; mud additives for petroleum excavation; and food additives. Examples of the water-swellable polymer include: crosslinked polyacrylate salts; saponified products of acrylic acid ester-vinyl acetate copolymers; modified products of crosslinked polyvinyl alcohol; crosslinked products of partially neutralized polyacrylate salts; crosslinked products of isobutylene-maleic anhydride copolymers; and starch-acrylic acid grafted polymers. These are, for example, used for: absorbing agents for sanitary materials such as sanitary articles and disposable diapers; water-holding agents for agriculture, forestry, horticulture, and tree planting; and food-freshness-retaining films. Demand therefor is going to further enlarge in the future.
There is, however, a possibility that either unreacted residual monomers in the above hydrophilic polymers or water-soluble high molecules in the water-swellable polymer might flow out due to external pressure or with time to contact human skins or be absorbed into human bodies. Thus, serious problems might occur according to uses. In addition, there is also a danger that the above monomers or high molecules might be disposed of as wastes to flow out to the environment and then mingle into drinking water and so on again. Therefore, in recent years, demands to reduce the residual monomer content in the hydrophilic polymer or the water-soluble high-molecular content in the water-swellable polymer are getting greater and greater.
As to processes to reduce the residual monomer content in the polymer, generally, for example, there are the following proposed processes: a process in which ammonia or an amine (JP-B-33-002646 and JP-A-50-040689) or a sulfite or hydrogensulfite salt (U.S. Pat. No. 2,960,486 and JP-A-55-135110) is added to cause addition reactions of these compounds to the residual monomer, thereby reducing the residual monomer content; and a process in which a low or high temperature decomposition type polymerization initiator (JP-B-50-044280, JP-A-59-133205, and JP-A-53-141388) or a combination of a redox catalyst and an azo initiator (JP-A-50-096689 and JP-B-47-026430) is further used to polymerize the residual monomer, thereby reducing the residual monomer content. However, the addition of ammonia, the amine, or the sulfite or hydrogensulfite salt may be effective in the reduction of the residual monomer content, but, in the case where the amount of these compounds as added is small, there is no effect, and further, there are problems of toxicity of these additives themselves. In addition, the process, involving the further use of the catalyst, merely has insufficient effects.
On the other hand, in the case where a hydrogel polymer is obtained by aqueous solution polymerization of a hydrophilic monomer and then dried to produce a hydrophilic polymer, for example, there is a known process to reduce the residual monomer content comprising the step of drying the hydrogel polymer in which the hydrogel polymer is dried in the temperature range of 80.about.250.degree. C. while being caused to contact water-vapor-mixed air having a dew point of 50.about.100.degree. C. (JP-A-01-026604). This process, however, might need too large a drying oven. In addition, for example, there is a proposed process in which: the hydrogel polymer is partially dried, and the temperature of the gel is then raised by microwave irradiation, thereby attempting to reduce the residual monomer content (JP-A-05-209010). However, this process cannot be said to be an industrially fit process, in that either the effect to reduce the residual monomer content or the enhancement of the absorption capacity may be seen, but, in the case of the water-soluble polymer, the formation of insoluble products or the reduction of the molecular weight is involved, and further in that, in the case of the water-swellable polymer, the properties thereof are much inferior to those of the hydrogel polymer such that the increase of the water-soluble content is seen. In addition, the microwave irradiation in the drying step involves local heating of materials to change the properties of the hydrogel polymer, and further, has many problems to overcome in the future in respect to the safety of the microwave itself, the restriction of materials in the vicinity of the microwave irradiation, and high cost of devices, therefore, is not practical in the current state.
In addition, there is also a proposal of a process in which the water content is reduced from 40.about.70 weight % to 10.about.35 weight % at 60.about.120.degree. C. under vacuum (JP-A-05-310806). However, the drying under vacuum involves the device cost up of the drying oven and needs a long drying time, therefore, is not practical.
As a relating technique, there is also a proposal of a process in which the hydrogel is treated with water vapor at the solid content of 20.about.60 weight % and the temperature of 50.degree. C. of the hydrogel to increase the solid content by up to 30 weight %, and then conventionally dried, thus attempting to enhance the absorption capacity (JP-A-03-076719). However, this process involves the increase of the water-soluble content as well as the increase of the absorption capacity, and greatly deteriorates the properties, therefore, cannot be said to be a fit process.